How to Determine the Boom Length on a Recumbent

Recumbent bicycles allow the rider to pedal from a reclining position with feet and legs basically horizontal. An increasingly common sight on the roadways today -- though still an object of curiosity to many -- recumbents actually date back to the mid-19th century with the introduction of the Macmillan Velocipede. Recumbent bikes can be generally divided into long wheelbase (LWB) and short wheelbase (SWB) categories. Riders of long wheelbase bikes tend to sit more upright, approximately at eye-level with cars, as the pedal cranks are located slightly below hip level. SWB riders are more severely reclined, have a low aerodynamic profile, and are less visible in traffic.

Things You'll Need

  • Measuring tape
  • Hex wrenches
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Instructions

    • 1

      Determine the wheelbase style of the recumbent bike. LWB models have a longer span between the front and rear wheels, and the fixed pedal crank is located behind the front wheel. SWB bikes have a shortened wheelbase, with the pedal crank located on a telescoping boom extended out in front of the front wheel. LWB models accommodate the rider's length -- known as the X-seam measurement -- by changing the position of the seat; SWBs adjust the telescoping boom length to move the pedal crank in or out.

    • 2

      Determine the correct X-seam measurement. Sit flat on a floor with your shoes off and your back against a wall. Extend both legs out before you. Have an assistant measure the distance from the wall to the bottom of your heel. X-seams of 38 inches to 42 inches are generally considered a short frame and boom setting. X-seams of 43 inches to 46 inches are generally considered medium, and 47 inches to 50 inches are a large frame and boom setting.

    • 3

      Release the two hex nuts in the clamp securing the telescoping boom and slide the boom in or out to the appropriate marked position of short, medium or long. Have the spring-loaded chain idler compensate for the change in chain length caused by repositioning the pedal crank. Tighten the clamp hex nuts to secure the boom.

    • 4

      Re-size the chain length if boom changes outside the standard small, medium and large adjustment range are necessary. Derailleur cables may also need to be lengthened or shortened.